Metal separation



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A. E. VANDERCOOK METAL SEPARATION Filed March 6. 1934 All ' lan. s, 1935.

Jan. 8, 1935. Afa. vANDl-:RcooK METAL SEPARATION Filed March 6, 1934 2 Sheets-Sheet 2 QuZmkQ L U ATTORNEY `Patented Jan. 8, 1935 UNITED STATES PATENT OFFICE METAL SEPARATION V Albert E. Vandercook, Sacramento, Calif., assigner to National Mining and uction Company,

Sacramento, Calif., a corporation of California Application March 6, 1934, Serial No. 714,349

15 Claims.

The invention relates to metal separation and particularly to the cyanide process of recovering precious metals such as gold.

'Among the objects of the invention are: 1. to provide a direct flow decantation process wherein the pulp is not clas'sed for separate treatment of the sand and slimes; Zuto shorten the time 'required for sending the major portion, as Well as the whole, of the values to precipitation; 3. to obtain bullion free from matte or sulphocompounds; 4. to discharge substantially all of the sulphides in the tailings, .without using lead salts or other compounds requiring constant close supervision of the amount of such compound necessary; 5. to automatically generate or create in the solution the amount of compound necessary to pass substantially all of the sulphides'., in linsoluble form to the tailings; 6. to make the `total recovery from the pregnant solution without eiecting a portion of the recovery by amalgamation.

` The important part that oxygen takes is facilitating'the solution of gold in the lixiviating solution is well understood. It has heretofore been proposed to separately treat the sands and the slimes, allowing each to stand in its solution for a varying length of time up to several days, to permit the oxygen of the air to act on the solution. The prior process may be called the batch procesa-to distinguish it from the present procy ess wherein the solution passes in a continuous manner through the apparatus in the same direction as the pulp, without separation of the sands andthe slimes, whereby a considerable saving is effected, not only of time but also iii the amount of water required.

It has heretofore been proposed to render the cyanide process continuous. According to pages 1043 to 1045 of the Hand Book of Metallurgy by Dr. Schnabel, published 1921, the only continuous process which has been used extensively is the Dorr continuous .counter-current decantation process, which is generally employed as an allsiiming process in which the ore is crushed in cyanide solution. In that case, clear water and barren solution lproceed through a series of thickeners in a direction opposite to the ilow of the pulp there-through. The overilow from the rst o f the secondaryvthickeners is` partly short circuited onit'self through 'a series of agitators, the major portion being supplied to the tube mill and from thence to the primary thickener, the overflow from which goes to the precipitation (c1. vs -iss) tank. Consequently, whatever values are recovered by the secondary thickeners, can only reach the precipitation tank by way of the primary thickener after passing .through the tube mill. As distinguished from the Dorr process, my invention provides for the differential Withdrawal of the pregnant solution from two separated points along the path of` travel of the pulp solution, the first of these points being the overiloW from .the primary thickener (without previous recovery of amalgam and without previous agitation except that inherent in the ball mill and classier) where concentrated pregnant solution is withdrawn to precipitation, and the second point being from one of the secondary thickeners, such as the rst one, where the pulp has previously been subjected to agitation 'and to dilution from the barren solution resulting from precipitation. Also a part of the overflow from the selected one of the secondary thickeners and the overilow from a subsequent secondary thickener is regenerated to form aerated mercury cyanide solution which is returned to the ball mill. In this way, the values are returned quicker to precipitation, than heretofore. My process may be termed the direct ow decantation process to distinguish it from the counter-current process above mentioned.

My'direct-flow continuous process is made possible, partly by the fact that the oxygen-necessary to facilitate the chemical reaction is added to the solution by aeration, thereby avoiding the prolonged treatment heretofore required in the. batch process.

'The small usage of water, according to the present invention, results from the recovery of all of the solution except the small amount necessary to move out the pulp. Also in the nal stages, the solution is diluted with freshwater and barren liquid from the precipitation system, whereby the cyanide and value loss is kept very low.

An important part of the invention resides in the chemistry of the recovery process. According to this feature of the invention, the ore in the ball mill is ground in the presence of mercury cyanide solution which has been aerated, hereafter called the solution, in order that the `ore while being ground may be subjected to this solution. 'Ihe important part that both mercury and oxygen have in this process are explained below.

Alkali cyanides in the presence of oxygen .as'

an oxidizer, dissolve mercury to form mercurio cyanide soluble in water and cyanides.

Mercury cyanide is soluble in eight parts of water at 15 C. It is also soluble in sodium and potassium cyanide solution. When sodium or potassium cyanide solutions, of the strengths used in metallurgical plants treating gold and silver ores, is saturated with atomized air, mercury can be dissolved by the plant solutions.

It is conceded by most metallurgists that oxygen is indispensable for the solution of gold and silver from their ores by the ordinary cyanide process solutions, and that it acts directly or indirectly.

In the mercury cyanide process, oxygen is absolutely necessary and performs three very important functions. First, as a depolarizer and an oxidizer of mercury which oxide is being dissolved by sodium cyanide, also in the same manner while gold and silver are being dissolved from the ores.

Second, as an oxidizer of any reducing agents that may start to forml in the working solutions.

Third, as a most important oxidizing agent in gaseous form to replace all the reducing hydrogen gas released into the working solutions during the precipitation of the metals.

These three functions of oxygen are of the utmost importance.

The function of the mercury in mercury cyanide solutions is five-fold.

First, as a disintegrator of the gold particles. It is easily proven that gold obtained from panning ore when mercury cyanide solution is added is immediately coated with mercury and starts to disintegrate the particles of gold.

Second, as an assisting agent for the more v rapid solution of gold and silver in their ores.

v This has been repeatedly proven in comparative tests in the laboratory and in actual plant operation. y

Third, as a precipitant for alkaline sulphides that start to form in the working solutions of a plant. Mercury has a greater afnity for sulphur than for cyanide and `Yforms with sulphur insoluble HgS and so keeps the working solutions from becoming foul with interfering elements.

Fourth, improve neness of bullion recovered by keeping sulphids and other basemetals out of precipitate.

Fifth, economy due to eliminating necessity for adjunctive chemicals, and to small amount of mercury itself which is required.

The action of mercurio salts in cyanidation is well known. However added, mercuric chloride, or otherwise, mercury in cyanide solution probably functions as the compound-KzI-Ig(CN)4. 'I'he chemistry of this salt is 'well known, and it functions in solution of gold and silver without the intervention of oxygen. The innovation in my invention is not in the use of any mercury salts, but in the method of its formation and introduction into the cyanide' solution; that is, the use of mercury in the cycle in metallic form and the dissolving thereof in alkali cyanide to form the desired double cyanide. This reaction, I have established. In the passageof the usual working mill solution of alkali through a mercury cyanide generator, a small amount of mercury is dissolved and is present in all of my working solutions at all times.

Oxygen is necessary4 for this solution effect,

and the amount dissolved can be controlled by introducing or excluding a regulated amount of dissolved oxygen. While, as stated above, dissolved mercury.is present in all my solutions, the amount dissolved isnever great; -for only rarely are even traces of. mercury found upon the zinc shavings or zinc dust in precipitation. The usual amount found to be present in solution is of the order of .01% to .015% and less.

This solvent action of alkali mercury cyanide is a very efcient aid in extraction, particularly upon some of the more diflicult soluble silver minerals, and it has been found materially to shorten the time of treatment of complex gold ores.

y Another function of dissolved mercury in solution is that for which salts of leadare usually employed, viz., the removal of soluble .sulphide sulphur by the precipitation of the very insoluble mercurio sulphide. The use of lead salts in cyanidation is usually haphazard and most difcult of intelligent control. In my system, mercury is always present in our solutions and a regulated amount of aerated mercury cyanide is formed automatically as needed.

In the development of this process and system, opportunity was offered to overcome many difculties found in usual cyanide practice and this resulted in most cases in complete extraction during crushing, instead of long agitation, therebyL shortening the time cycle. Heating of solutions as with silver ores is not required.

The solubility and eect of metallic mercury in cyanide solutions First. A .25% or 5 lb. solution of potassium cyanide was left in contact with mercury in agitation with free access of air to the surface for 15 hours. and showed a solubility of mercury equal to .019%. A .3% or 6 lb. solution of sodium cyanide was similarly treated and gave mercur .022% in solution.

These simple experiments were merely qualitative, other factors such as temperature, dissolved oxygen Were not taken under control.

. The solubility in some cases, is affected by difference of concentration l and temperature Hg-i-iKCn-I-HO+O=KzHg(CN4 +2KOH. AThis is analogous to the Elsner's Equation for' gold and silver. Julian and Smart give, Hg+4KCn| 2H2O=K2Hg(CN4) +2KOH|H2. Preference is for the former statement of the reaction-in either case oxygen is essential, either entering directly into the reaction or as combining with evolved hydrogen, removingl it from occlusionso that the reaction may proceed. The necessity for dissolved oxygen cannot be too strongly emphasized. 'I'he amount of mercury going Ainto solution in a given time is directly controlled by the amount of oxygen available and the oxygen derived from pure water has no significance when one considers conditions met in ordinaryvpractice.. The eiect of. the dissolved saltsand solids in suspension by'increasing viscosity and by lchemical reactions, decrease the solubility of oxygen, and these factors vary enormously in practice. Consequently, the amount of 4mercury going into solution under working conditions is slight.

VSecond.Desu1phurizing action of merury in solutionz--The reaction whereby the sulphur of sulfide minerals of ores enter into solution in' Cil working solutions are obscure and exceedingly complicated, although the deleterious effect of alkali suldes is well known, they are powerful reducing agents and cyanicides. K2S+KCn+ H2O+O=KCnS+2KOH; also they precipitate dissolved silver as silver suldes and may under certain conditions precipitate gold and silver. In ordinary practice, soluble lead salts are added to remove alkali sulfides, by the precipitation of leadsuldes. This practice is not subject to exact control, and is governed usually by guess. Mercury salts are equally effective as lead salts.

My solution has an efficient desulphurizing action at the moment of formation of alkali suldes, with advantages over former practice.

Third. Dissolving power of mercurio cyanide K2Hg CN 4 on gold and silver.

This eiect is well established, and takes place without the intervention of oxygen.

In practice it is probably more marked as an aid in the dissolving of silver.

In the non-continuous cyanide process in cornmon use, in spite of the recognized advantages of recovering the coarser metallic particles of gold and silver, upon which the dissolving effect ofcyanide is slow, when crushing in solution, amalgamation is not largely practised. There is always a mechanical loss of mercury and of amalgam from the plates which finds its way in part into the tails. In my system such loss is impossible, as the coarse or heavy gold or silver particles remain in the grinding circuit until dissolved.

Another and more serious disadvantage in the noncontinuous cyanide process in common use is the presence of metallic copper. action of copper on cyanide is well known, a source of loss from beginning to end by consumption of cyanide, by deterioration and solution of plates by reducing action, (loss of oxygen), by consumption of zinc, by introduction of copper into precipitates and bullion. In this system the use of copper as metal or alloy is entirely avoided.

Mercury not so consumed by alkaline suldes is free to aid as a solvent, whilelead ,salts have no such action and are rather'deleterious than otherwise. The necessary and uncertain excess of lead salts tend to be lost as sulphate or carbonate, serving no useful purpose.

Nomercury is found on the zincs under normal working conditions. The mercurio cyanide K2Hg(CN) i performs its functions as formed and does not accumulate iii/the solutions. The consumption of mercury is light and the cost per ton insignificant as -only to '7 lbs.` is required for 25,000 tons of ore treated.

The above described solution reaction throughout the cycle has rendered bullion free from matte or sulpho compound. The precipitate requires no preliminary treatment, such as an acid treatment, and can be fluxed and melted direct into bullion.

The presence of mercury in the solution is also beneficial in assisting the precipitation of the precious metals.

In carrying out the invention, the various pieces of apparatus employed in the `recovery The deleterious.

process are arranged approximately in a circular array, with a pilot house in the center thereof, the various pieces of apparatus being arranged for the most part at a levelequal to or less than the level of the pilot house,l whereby a single operator in the pilot house can Watch substantially all of the pieces of apparatus in operation. Furthermore, remote control switches for operating the various motors in the system, together with indicators for indicating the height of the liquid in a stock tank or other tanks, are all brought together on a common switchboard at a convenient point in or adjacent the pilot house, thereby further facilitating the control of the apparatus by a single operator. The use of this continuous process, with the convenient arrangement of apparatus above described, makes it possible for a single operator to run a mill handling 150 tons or more of ore per day.

I have discovered that a major portion of recovery, viz. 90%, is had by the time the pulp leaves the classifier and I immediatelyvdecant or thicken the pulp and pass the pregnant solution to precipitation. A further advantage of not agitating prior to the i'lrst thickening is to avoid emulsifying any carbon present, as with my arrangement the carbon oats on the surface of the rst thickener and can be removed by skimming.

For further details of the invention reference may be made to the drawings, wherein: Y

Fig. 1 is a flow diagram somewhat in vertical aspect, the path of the pulp being indicated by dashed lines. Y

Fig. 2 is a flow diagram, in horizontal aspect, indicating the general arrangement of the apparatus around the pilot house.

Referring to Fig. 1, the ore Afrom a car is dumped. on grizzly 1, the ore passing through the grizzly being received by a belt conveyor 2, the ore retained by the grizzly being supplied to a gyratory crusher 3 which delivers crushed ore to the belt 2 as will be readily understood. Lime is added to the crushed ore on belt 2 from the'container 4. The belt 2 delivers the mixture of lime and crushed ore' to the ball mill 5, to both the heads and the tails of which are continuously supplied the solution, l. e. aerated mercury cyanide solution, from a stock tank 6. A launder 8, if desired, may contain riiiles to catch large particles of gold or silver and retain the same until they are removed or rlnally dissolved by the solution, instead of returning them to the ball mill.

The output from the ball mill 5 is supplied to a Dorr classifier 7 which returns the sands to be reground by the ball mill 5, the pulp, that is slimes, or sand and slimes which have been ground to a suitable flneness are supplied by gravity through the launder 9 to the number 1 primary thickener.

Pulp from the bottom of primary thickener number 1 is pumped by diaphragm pump 10 to the top of agitator 11. Pulp from the top of agitator 11 flows by gravity to the top of agitator 12. Pulp from the top of agitator12 flows to the top of secondary thickener number 2 and from the bottom of latter pulp is pumped by diaphragm pump 13 to the top of another secondary thickener number 3. of thickener number 3 pulp is pumped by diaphragm pump 1` t' the tailings discharge 15.

The pregnant solution flows by gravity from thickener number 1 to the clarifying filter 16 to which is also supplied pregnant solution from From the bottom P the thickener number 2. The filter 16 clarifies the overfiow solution from the thickeners number 1 and number 2, removing for example talc. The filter 16 may be of the well known leaf type.

This prepares the solution for the precipitation of gold and silver by zinc dust or the like. The clarified solution is drawn from lter 16 by pump 17 which discharges to an indicator tank 18, the discharge from the latter being pumped by a vacuum pump 19 and then by gravity to a zinc dust emulsier 20.` The emulsier 20 supplies in parallel a large number of collecting bags indicated at 21. The barren liquid from the precipitation or collecting bags 21 flows by gravity to a device which forms an important part of this invention and which I have termed a mercury generator, indicated at 22, to receive the barren liquid from bags 21 and also indicated at 23 to receive the overflow from thickeners number 2 and number 3. This mercury generator is of the construction shown in my Patent No. 1,156,946 and comprises a tank 24 having rotatably mounted therein a rotatable pipe 25 having oppositely extending discharge nozzles 26 and 27 communicating with the axial pipe 28 which receives the discharge from the pump 29, the inlet to rthis pump being taken from the bottom of theA tank 22. Air is admitted to the inlet of pump 29 by the valve 30, to aerate the solution. The discharge from pump 29 through the nozzles 26 and 27 causes the arm 25 carrying these nozzles to rotate and distribute the discharged liquid over the surface of a pool of mercury 31 in the bottom of the tank 22, to thereby form aerated mercury cyanide solution.

According to my prior patent above mentioned, and also according to my Patent No. 1,135,080, I proposed to supply to the mercury generator the solution together with sand and slimes, whereas according to the present invention the mercury generator operates only on clear solution from which the sand and slimes have been removed.

Furtherfore, according to the present invention the mercury generator is to better advantage connected into the system at points other than the points disclosed in my previous patents.

In accordance with the chemistry heretofore explained, it will be understood that generators 22 and 23 serve as sources of aerated mercury cyanide solution. The output from the mercury generator 22 discharges partly at the pulp input to agitator 11 and partly at the( pulp input to agitator 12, to dilute the solution at these points and to subject the same to the benefit of the aerated mercury cyanide solution as heretofore explained.

The mercury generator 23 receives the overflow from thickeners number 2 and number 3 and furnishes aerated mercury cyanide solution to the sump 32 from which it is drawn by pump 33 to the stock tank 6, in order that the ore in the ball mill 5 at the very outset may be treated with aerated mercury cyanide solution with the benefits before mentioned.

Solutionis supplied to the mercury generators 22 and 23, from precipitation,4 and from the thickeners numbers 2 and 3 respectively, at the return of about 60 gallons per minute. The area of the mercury surface in each of the generators 22 and 23 is about 700 square inches. The area of the air inlet controlled by valves and 38 is about one-eighth inch in diameter. The speed of the pump 29 and of the corresponding pump for generator 23 is about 1200 R. P. M. The

amount of air drawn in can be controlled by regulating the valves 36 and 37 in the inlet to the pumps such as 29. The relation of the elements is such that there is a fraction of 1 suon as .01% to .015% and less of mercury in all parts of the system.

yFresh Water from a pipe 34 is supplied to the outlet of pump 13 in order .to replace the small amount of Water required to move the tailings to the discharge 15, Yand to'further dilute the solution carrying the pulp, the initial dilution being obtained in the agitators 11 and 12 by the barren liquid from the generator 22, as above explained. Also, from time to time, sodium cyanide is added to the system, for instance in generator 23, to replace the cyanide lost in the tails. Safety or overflow pipe lines 39, 40, 41, and 42 are provided as indicated.

A plan view of the apparatus is indicated in Fig. 2. Except for the stock tank 6, which is elevated, and except for the sump 32 in the basement of the mill, the various pieces of apparatus are arranged generally at the level of pilot house which is centrally located with respect thereto. Remote control switches for the motors driving the various pumps and for the motor driving the ball mill 5, as Well as indicators showing the level of the liquid in the tanks 6 and 17, are brought to a common point in or on the pilot house, whereby a single operator may readily control all of the apparatus. lIhe pilot house is completely surrounded with glass windows to keep out the noise of the mill while permitting the attendant to inspect all of the apparatus from a central point within the pilot house.

Practical experience with the process herein disclosed has shown that a recovery of the order of 97% in gold can be made from low grade ore assaying from $5.00 to $7.00 per ton, While handling 100 tons per day. 'I'he strength of the cyanide solution used in the stock tank is. about 1.1 lb. per ton of solution. The time of the cycle is 91/2 hours. The ore is ground so that about 97% is -100 mesh and about 75% is -200 mesh. The consumption -of sodium cyanide is about 1/2 pound per ton of ore including tailing loss and the consumption of zinc dust about .025 pound per ton of ore treated.

For the first three .weeks this mill was in operation, the values in the bags plus the tailing loss Was approximately two-thirds the value of the mill heads, showing, as was later verified by results, that coarse gold not readily soluble, was remaining in the grinding circuit. It was not until twenty one days after starting the mill that the recovery plus the tailings loss was as large as the values in the mill heads. In fact, after the three weeks period had elapsed, the recovery in the bags was frequently higher than the value of the mill heads, so that the average results over operations for 6 or 8 weeks checked out properly. The recovery therefore is preferably made wholly from the pregnant solution without attempting to recover the coarse gold by amalgamation on a copper plate as the latter has disadvantages pointed out above.

It will be apparent that various modification may be made in the process and apparatus herein disclosed without departing from the spirit of the invention.

Having thus described the invention, what is claimed as new and desired to secure by Letters Patent is:

I claim! 1. The steps in the method of metal separation which comprises grinding ore in the presence of aerated mercury cyanide solution, forming said mercury cyanide solution by aeration of cyanide solution in the presence of metallic mercury, and in controlling the amount of aeration to control the amount of mercury in solution whereby the sulphids present in the pulp pass substantially wholly into the tailings, and recovering the values in solution substantially wholly by lixiviation.

2. The steps in the method of metal separation which comprises classifying wet pulp in the presence of aerated mercury cyanide solution, forming said mercury cyanide solution by aeration of cyanide solution in the presence of metallic mercury, and in controlling the amount of aeration to control the amount \of mercury in solution whereby the sulphids present in the pulp pass substantially wholly into the tailings, and recovering the values in solution substantially wholly by lixiviation.

3. The steps in the method of metal separation which comprises agitating wet pulp in the presence of aerated mercury cyanide solution, forming said mercury cyanide solution by aeration of solution substantially free from pulp, and in controlling the amount of aeration whereby the sulphids present in the pulp pass substantially Wholly into the tailings, and recovering the values in solution substantially wholly by lixiviation.

4. Improvement in the cyanide process of metal separation which comprises grinding ore in the presence of aerated mercury cyanide solution, directly separating the ground ore from the pregnant solution without previous agitation -after grinding, treating the pregnant solution to recover the metal dissolved therein, aerating the resulting barren solution in the presence of mercury to form aerated mercury cyanide solution and in subjecting the ground ore to the last mentioned solution.

5. The continuous cyanide process of metal separation which comprises grinding ore in the presence of a continuous supply of aerated mercury cyanide solution, continuously thickening and then agitating the resulting pulp, continuously removing clear pregnant solution from the pulp thus treated and precipitating metal therefrom, continuously passing the tailings, with sand and slimes together, to a tailings dump, returning the barren liquid resulting from said precipitation, to increase the uid content of the pulp solution after initial thickeningLadding enough fresh water to replace the fluid required vto move the tailings to a point of discharge and to further reduce dissolved values in the thickened pulp, and regenerating the overflow from said thickening to provide said aerated mercury cyanide solution in the presence of which said ore is ground.

6. Improvement in the cyanide process of metal separation which comprises grinding ore in the presence of aerated mercury cyanide solution, decanting pregnant solution therefrom, precipitating metal `from said pregnant solution to form barren solution, diluting pulp resulting from said decantation with said barren solution to form diluted pregnant solution, decanting said diluted pregnant solution partly for said precipitation and partly for the formation of aerated mercury cyanide solution, treating the decanted liquid to form said aerated mercury cyanide solution and supplying said aerated mercury cyanide solution for said grinding.

'7. Metal separation apparatus comprising the combination of means for supplying pulp in cyanide solution, an agitator therefor, means lfor decanting clear liquid from said pulp, an aerated mercury cyanide generator, means for supplying said clear liquid tosaid generator, said generator comprising a. tank, a pool of mercury in said tank, a pump for aerating the liquid in said tank and returning the same over said pool of mercury, means for withdrawing aerated mercury cyanide solution from said tank and for adding the same to the pulp supplied to said agitator.

8. Metal separation apparatuscomprising the combination of a precipitation unit, an aerated mercury cyanide generator, an agitator, means for supplying filtrate .from said precipitation unit to said generator, and means for supplying aerated mercury cyanide solution from said generator to said agitator.

9. Metal separation apparatus comprising the combination of a plurality of thickeners, a precipitation unit, an aerated mercury cyanide generator, means for supplying overflow from said rst of said thickeners partly to said precipitation unit and partly to said generator, means for supplying overflow from said second thickener to said generator, means for diluting with fresh water the pulp from said second thickener, and a sump tank in circuit with said generator.

10. Improvement in the cyanide process -of metal separation which comprises grinding ore in the presence of aerated mercury cyanide solution, successively thickening, agitating, and thickening the pulp and thereafter again thickening the pulp, said last mentioned thickening being in the presence of fresh water, and thereafter passing the pulp to a tailings discharge, withdrawing the overflow from said iirst mentioned and second mentioned thickening for precipitation of the metal, passing the resulting barren solution to dilute the pulp at said point of agitation,v and aerating the overflow from said third mentioned thickening in the presence of mercury to form the aerated mercury cyanide solution in the presence of which the ore is ground.

l1. Process according to claim wherein said barren solution is aerated.

12. Process according to claim 10 wherein said barren solution and the overflow from said third mentioned thickening are aerated in the presence of mercury to form aerated mercury cyanide solution.

13. The direct flow decantation cyanide process of metal separation which comprises grinding ore in the presence of aerated mercury cyanide solution, decanting the pregnant solution from the pulp, precipitating the metal from the pregnant solution, aerating the barren solution in the presence of mercury, mixing the pulp with said last mentioned solution and thereafter subjecting the same to agitation and decantation, and aerating in the presence of mercury the overflow from said last mentioned decantation to provide the solution for Saidgrinding.

14. The steps in the method of metal separation which comprises aerating cyanide solution Substantially free from pulp in the presence ofl metallic mercury to form mercury cyanide which is soluble in cyanide solution, said aeration also being carried out to a point to provide an excess of oxygen which facilitates the solution of4 precious metal or metals in the cyanide solution, controlling the amount of said aeration to control the amount of the compound of mercury which is formed in solution, whereby sulphides which would otherwise be soluble are prevented from going into the solution due to the presence ofmercury in saidsolution, said sulphides therefore passing in solid form substantially wholly 5 into the tailings, and recovering the values substantially wholly by lixviation in said solution .without substantial recovery by amalgamation.

15. Metal separation apparatus comprising the combination of apparatus for dissolving metal in and recovering metal from cyanide solution, and means in circuit therewith for continuously generating aerated mercury cyanide solution from cyanide solution substantially free from pulp.

ALBERT E. VANDERCOOK. 

